Collaborative Research: Subduction Zone Segmentation over Multiple Seismic Cycles, South-Central Chile

俯冲带断层的可能分段是目前地震发生的物理学和地质学以及未来地震危险性评估中最重要的问题之一。未来的大地震将如何沿着断层排列？个别地震破裂是如何以及为什么会停止的？俯冲带断层形成了平行于世界上许多国家海岸线的构造板块边界。近几十年来，仪器和建模的进步已经对断层行为产生了重要的见解，但这些测量只跨越了大地震之间的时间间隔的一小部分。该研究项目的目标是利用保存在沿海地貌和沉积物中的跨学科证据阵列，调查俯冲带断层的分段是否在数百年至数千年的多个地震周期中保持不变。该油田位于智利中南部的阿劳科半岛海岸，跨越了1960年5月22日（Mw 9.5）和2010年2月27日（Mw 8.8）全球历史记录中最大的两次俯冲带地震的破裂带之间的边界。在2010年之前，智利中南部本项目的研究区域是沿秘鲁-智利海沟沿着最被接受的硬断层段边界之一;据信，它是几个历史断裂的边界，包括1960年的智利大地震。2010年2月的智利地震对这一假设提出了质疑，当时破裂延伸到该边界以南，仅与公元1835年的前一次地震部分重叠。过去4000年来地震的地质证据，频繁的大型俯冲带地震，以及智利中南部异常长而全面的地震历史记录，使该地区成为解决断层分段基本问题的天然实验室。本研究计划将测试有关地震行为和海岸反应的多个假设：（1）多个地震周期的断层破裂可能终止于固定（硬）、可变（软）或随机边界，或可能包括多个区段;（2）大俯冲带地震将产生海啸存款，同时发生陆地水平的突然变化;（3）全新世海岸演化反映了相对海平面的适度净变化，大震时的垂直升降在地震间隔期发生了很大的逆转。为了解决这些假设，研究小组将利用地貌学、沉积学和微化石分析，重建与多个地震周期相关的垂直陆地水平变化和海啸历史。他们将描述保存沉积序列中陆地水平变化和海啸沉积物的最佳环境条件，并分析这些陆地水平变化对沿海景观的影响。俯冲带断层的断裂造成了世界上一些最大的地震和海啸，例如2011年日本发生的灾难性地震，造成数千人死亡，摧毁了沿海城市。最近在2004年苏门答腊、2010年智利和2011年日本发生的破坏性地震中，断层断裂的不可预见程度和由此产生的地震震级，突出了这一调查对评估其他类似地质环境中未来地震危险的实际重要性。该项目将建立在与智利科学家现有的强有力的合作基础上。研究结果将通过与智利的联系为地震和海啸风险地图的绘制提供信息，我们还将安排关于地震和海啸危险的公开介绍。一般的概念将推进知识的地震行为的时间周期，超过了相对较短的时期内的仪器记录。本项目得到了地球科学司和国际科学与工程办公室的构造学计划和地貌学与土地利用动力学计划的支持。

The possible segmentation of subduction zone faults currently presents one of the most significant questions to the physics and geology of earthquake occurrence and the assessment of future earthquake hazards. How will future large earthquakes be arrayed along a fault? How and why will individual earthquake ruptures stop? Subduction zone faults form the tectonic plate boundaries that parallel the coastlines of many countries around the world. Advances in instrumentation and modeling in recent decades have produced important insights into fault behavior, yet these measurements span only fractions of the time interval between major earthquakes. The goal of this research project is to use an interdisciplinary array of evidence preserved in coastal landforms and sediments to investigate whether segmentation of subduction zone faults is maintained over multiple earthquake cycles spanning hundreds to thousands of years. The field are, the Arauco Peninsula coast of south-central Chile, crosses the boundary between the rupture zones of two of the largest subduction zone earthquakes in the worldwide historic record, on May 22, 1960 (Mw 9.5) and February 27, 2010 (Mw 8.8). Prior to 2010, the study area for this project in south-central Chile was one of the most accepted hard fault segment boundaries along the Peru-Chile trench; it was believed to bound several historical ruptures, including the Great Chilean earthquake of 1960. The Chilean earthquake in February 2010 called this hypothesis into question, when the rupture extended south of this boundary and only partially overlapped with the previous earthquake in AD 1835. The combination of geological evidence of past earthquakes over the last 4000 years, frequent large subduction zone earthquakes, and an unusually long and comprehensive historic record of earthquakes in south-central Chile makes this region a natural laboratory to address fundamental questions about fault segmentation. This research project will test multiple hypotheses regarding earthquake behavior and coastal response: (1) fault ruptures over multiple earthquake cycles could terminate at fixed (hard), variable (soft), or random boundaries, or could include multiple segments; (2) large subduction zone earthquakes will produce a tsunami deposit simultaneous with an abrupt change in land level; and (3) the Holocene coastal evolution in this region reflects a modest net change in relative sea level in which the vertical uplift or subsidence during large earthquakes is largely reversed during the periods between earthquakes. To address these hypotheses the research team will reconstruct vertical land-level changes and tsunami history associated with multiple earthquake cycles using geomorphic, sedimentological and microfossil analysis. They will characterize the optimal environmental settings for the preservation of land-level changes and tsunami deposits in sedimentary sequences and analyze the impacts of these land-level changes on the coastal landscape.Ruptures on subduction zone faults have created some of the largest earthquakes and tsunamis in the world, such as the catastrophic earthquake in Japan in 2011 that killed thousands of people and destroyed coastal cities. The practical importance of this investigation for assessing future earthquake hazards in other similar geological settings was underscored by the unforeseen extent of fault ruptures and resulting earthquake magnitudes in the recent devastating earthquakes in Sumatra in 2004, Chile in 2010 and Japan in 2011. The project will build on strong existing collaboration with Chilean scientists. The results will inform the construction of earthquake and tsunami risk maps through established contacts in Chile, and we will arrange public presentations on earthquake and tsunami hazards. The general concepts will advance knowledge of earthquake behavior over time periods that exceed the relatively short period of the instrumental record.This project is supported by the Tectonics Program and Geomorphology and Land Use Dynamics Program in the Division of Earth Sciences and the Office of International Science and Engineering.